Method for making thin film transistors for a liquid crystal display

ABSTRACT

A method of manufacturing thin film transistors for use in a liquid crystal display which reduces the failures due to excessive leakage current. A silicon layer, first insulating layer, and gate electrode layer are serially formed over a transparent substrate. The gate electrode layer is patterned to form a plurality of gate electrodes and associated pairs of gate line electrodes, and the silicon layer is patterned into thin-film transistor regions. Then, a relatively thick second insulating layer is formed over the substrate, and contact holes are formed in the second insulating layer. Finally, a metal layer is formed and patterned over the second insulating layer and through the contact holes to connect each gate electrode with its associated pair of gate line electrodes; and to form source and drain electrodes. The thick second insulating layer provides increased spacing between the thin-film transistors and the associated gate interconnects, thereby reducing leakage current and failures due to excessive leakage current.

This is a continuation of application Ser. No. 08/493,746, filed on Jun. 22, 1995, which is abandoned upon the filing hereof; which is divisional of application Ser. No. 08/350,003 filed Nov. 29, 1994, now abandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to liquid crystal display panels. More particularly, the present invention relates to thin film transistors (TFTs) and a method for making thin film transistors having a gate electrode and gate line electrodes connected by means of a metal line in a thin film transistor formed of polysilicon.

(2) Description of the Prior Art

As contemporary image display devices, such as high definition television sets, have been developed, flat panel display devices have been much in demand. The liquid crystal display is representative of techniques used to make flat panel display devices. The liquid crystal display (LCD) contributes a combination of colorgenic features, low power consumption, and high speed performance that electroluminescence devices (EL), vacuum fluorescence displays (VFD), plasma display panels (PDP), etc., have failed to attain.

An LCD device may be characterized as an active device, or a passive device. The active device is superior to the passive device in performance speed, and in view angle and contrast readability. Thus, the active LCD device has been identified as being the most appropriate display device for high definition television sets which require resolution of more than one million pixels. As the importance of thin film transistors has been recognized in future commercial products, research activities directed TFTs have increased.

Research in the field of thin film transistors, which serve as electrical switches for selective driving of pixel electrodes in the liquid crystal display devices, has focused on improvements in the structure of the transistors, in the performance characteristics of the amorphous or polycrystalline silicon, and in the prevention of ohmic contact failures, opens and shorts, in the TFT electrodes. Each of these improvements ultimately enhances production yield.

Polycrystalline silicon thin film transistors are favorite elements used to form switching devices and peripheral driving circuits in liquid crystal display panels. Accordingly, a great deal of research has recently been devoted to thin film transistors for large scale LCDs which can be mass produced at low cost.

The most common materials used to form the substrate in a liquid crystal display panel is transparent glass produced in a low temperature fabrication process, and quartz produced in a high temperature fabrication process.

The steps of a conventional method used to manufacture a thin film transistor in a liquid crystal display panel are illustrated in FIGS. 1A to FIGS. 1F.

As shown in FIG. 1A, the formation of a conventional thin film transistor begins with depositing a silicon layer 12 on a transparent substrate 10. The silicon layer 12 is patterned, and an insulating layer 14 is deposited to overlay the upper surface of silicon pattern 12 and to surround both sides of silicon pattern 12, as shown in FIG. 1B.

Insulating layer 14, typically an oxide layer, may be grown in an ambient atmosphere of oxygen, or may be grown by a chemical vapor deposition.

Subsequently, a polycrystalline silicon layer 16 is deposited over the surface of substrate 10 on which the silicon pattern 12 is formed, as shown in FIG. 1C, and silicon layer 12 is doped by means of POCL₃ to lower it ohmic resistance. Subsequently, as shown in FIG. 1D, a gate pattern 16 is formed from the polycrystalline silicon layer, and ion implantation is performed.

As a result, ion implantation to form active drain/source regions is made into the entire upper surface of the resulting structure, except the region underlying gate electrode 16. An isolation oxide layer 18 is then deposited over the upper surface of the substrate on which the pattern is formed as shown in FIG. 1E. In order to achieve subsequent planarization of this layer, a borophosphosilicate glass (BPSG) may be used.

Activation of the implanted ions is carried out at a temperature of more than 800° C. After that, contact holes are selectively formed through isolation oxide layer 18, as shown in FIG. 1F, and a metal layer 20 is deposited to a predetermined thickness over and adjacent to the contact holes to form the source/drain electrodes, whereby the formation of a polycrystalline silicon, thin film transistor is completed. In order to reduce off current, the foregoing structure having light doped drain (LDD) may be used.

FIG. 2 depicts a top plan view of the conventional thin film transistor for use in a liquid crystal display device. Reference numerals designate the following parts: 12 a silicon pattern; 22 a contact; 23 a gate. A sectional view of the gate taken along lines A-A' has a structure appearing in FIG. 1C.

In a case where the cross-sectional view of the gate has the structure shown in FIG. 1C, a "weak point," exists within this structure, as denoted by II in FIG. 1C, which increases leakage current and degrades overall reliability of the TFT.

A 1991 article in the Journal of Electrochemical Society, Issue 138, page 802 by Troxel et al. entitled Enhanced Polysilicon Thin-Film Transistor Performance by Oxide Encapsulation, discloses the structure of the weak point. FIGS. 3A through 3C illustrate the teachings of this article. In FIG. 3A, a silicon layer 12 is deposited over a transparent substrate 10, and an oxide layer 14 of SiO₂ and a silicon nitride layer 17 are serially deposited over silicon layer 12. The oxide layer and silicon nitride layer are patterned into islands by means of a photoresist pattern.

The photoresist pattern is removed and thermally oxidized to form the oxide layer 14 shown in FIG. 3B. After the oxide layer 14 has been formed, the silicon nitride layer is removed. A gate layer 16 is deposited on the pattern of FIG. 3B to remove the weak point appearing in the conventional TFT.

U.S. Pat. No. 5,120,667 discloses a technique for forming a TFT having reduced leakage current and a simplified fabrication process. This technique is illustrated in FIGS. 4A to 4D.

To explain the above fabrication process briefly, a silicon layer 12, a thermal oxide layer 14 (or an oxide layer) and a gate polysilicon layer 16 overlay one another on a transparent substrate 10. The gate polysilicon layer 16 is doped and patterned into islands so as to form the structure shown in FIG. 4A. The insulating layer 19 is applied to the resulting substrate, as shown in FIG. 4B.

After that, the insulating layer 19 is etched by a reactive ion etching to reduce the insulating layer 19 to twin side walls covering both sides of oxide layer 14, as shown in FIG. 4C. A polysilicon layer 21 is deposited over the resulting substrate to yield the structure of FIG. 4D.

According to the above fabrication process, a silicon pattern is formed then patterned, an oxide layer is deposited, and an insulating layer is formed into side walls in order to prevent the side edges of the patterned silicon from being exposed.

SUMMARY OF THE INVENTION

The present invention is made in consideration of the above-mentioned problems.

It is an object of the present invention to provide thin film transistors for a liquid crystal display device and a method for making the thin film transistors which simplifies the fabrication process while preventing leakage current.

In order to achieve the above object, the inventive method for making thin film transistors for a liquid crystal display, comprises the steps of:

forming serially a silicon layer, a first insulating layer and a gate polysilicon layer to a predetermined thickness on a transparent substrate;

forming gate lines after doping phosphorus on the gate polysilicon layer and then performing ion-implantation;

applying a photoresist layer on the substrate including the gate lines to pattern the silicon layer into islands and etching the gate lines to form a gate electrode and gate lines to be with each space being of a predetermined width;

forming second insulating layers for separation of electrode lines on the substrate having the silicon patterns;

forming contact holes on the gate electrode, gate lines and silicon patterns; and

forming a metal lines so as to connect the gate electrode to the gate lines through the contact holes, simultaneously with forming a source-drain electrode on the contact holes.

As another aspect of the present invention, the inventive thin film transistor comprises:

a gate electrode formed on a transparent substrate;

gate lines formed to be separated from the gate electrode with a predetermined space;

silicon patterns formed to be cross the gate electrode and gate lines around the gate electrode;

contact holes formed on the gate electrode, gate lines and silicon patterns;

metal lines formed to have a narrower width than the gate lines on the gate electrode and gate lines so as to connect the gate electrode to the gate lines; and

a source/drain metal electrode connected to the source/drain contact holes.

Simplification of the fabrication process, and reliability of the resulting LCD may be ensured using the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F depict the steps in the manufacture of a thin film transistor for liquid crystal display device in accordance with a conventional art;

FIG. 2 is a plan view showing a structure of the thin film transistor for a liquid crystal display device in FIGS. 1A to 1F;

FIGS. 3A to 3C depict the conventional steps in the manufacture of a thin film transistor to prevent the leakage current caused in the silicon and gate of the thin film transistor;

FIGS. 4A to 4C depict a technique for forming a TFT as disclosed in U.S. Pat. No. 5,120,667.

FIGS. 5A to 5C depict the steps in the manufacture of a thin film transistor for a liquid crystal display device in accordance with the present invention, and

FIG. 5A is a sectional view showing that a silicon layer is deposited on a substrate;

FIG. 5B is a sectional view showing that a gate polysilicon layer is formed on an insulating layer;

FIGS. 5C-1 to 5C-2 are respectively a sectional view and a plan view showing that a gate pattern is formed on the insulating layer;

FIG. 6 is a plan view showing the pattern of the completed thin film transistor for a liquid crystal display device;

FIG. 7 is a sectional view taken along lines B-B' of FIG. 6; and

FIG. 8 is a sectional view taken along lines A-A' of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIGS. 5A to 5C (inclusive of FIGS. 5C-1 and 5C-2) depict the steps in the manufacture of a thin film transistor for a liquid crystal display device in accordance with the present invention. FIG. 5A is a sectional view showing a silicon layer 120 deposited on a substrate 100; FIG. 5B is a sectional view showing a gate polysilicon layer 160 formed over an insulating layer 140, and FIGS. 5C-1 and 5C-2 are, respectively, a sectional view and a plan view showing that a gate pattern 160' formed on insulating layer 140.

In the first step of the method of manufacturing according to the present invention, a silicon layer 120 is formed on a glass substrate 100 as shown in FIG. 5A. The silicon layer is formed to a thickness of about 800 to 1000 angstroms, but the thickness may be varied according to various design objectives.

Next, a first insulating layer 140 and a gate polysilicon layer 160 are serially formed over silicon layer 120 as shown in FIG. 5B, and the gate polysilicon layer is patterned into islands which form the gate electrode 160' shown in FIG. 5C-1.

Ion implantation is performed (170) on the pattern where the gate electrode is formed, as shown in FIG. 5C-1. FIG. 5C-2 shows a plan view of the pattern. The first insulating layer 140 is formed to a thickness of 500 to 1000 angstroms, and in the case of a high temperature fabrication process, a thermal oxide layer is thermally treated in an ambient atmosphere of oxygen. In the case of a lower temperature fabrication process, the oxide layer is formed by a photo-etching chemical vapor deposition method, or by a low temperature deposition. After the deposition of the silicon layer 120, the oxide layer 140 is formed to reduce impurities.

After the gate polysilicon layer has been deposited, the ion-implantation or doping with phosphorous with POCL₃ is performed to reduce resistance.

In order to form the thin film transistor as shown in FIG. 6, photoresist (not shown) is applied to the foregoing structure overlaying substrate 100 to pattern silicon layer 120 into islands. At this point, the gate electrode 160' is etched to have the shape shown in the plan view of FIG. 6, wherein gate line electrodes 160[D₁ ] and 160[D₂ ] are each formed with a space of predetermined width between the respective gate line electrodes 160[D₁ ] and 160[D₂ ] and gate electrode 161. The silicon layer 120 is patterned by reactive ion etching, and use of the photoresist which has been selectively applied to form the desired gate electrode pattern.

A second insulating layer 240 which separates the gate line electrode structure is deposited to a thickness of about 4000 to 6000 angstroms, or to a predetermined thickness on the surface of the resulting structure formed over substrate 100 where the pattern is formed, and the injected ion is activated.

As shown in FIG. 6, contact holes 220 are formed and a metal line 200 is formed through contact holes 220 so as to electrically connect the gate electrode 161 and gate line electrodes 160[D₁ ] and 160[D₂ ].

The source-drain metal electrode is formed on the contact holes on the side of the source-drain electrodes.

The thin film transistor formed through the above process, includes gate electrode 161 formed over transparent substrate 100; the gate line electrodes 160[D₁ ] and 160[D₂ ] formed on either side of gate electrode 161, each having a space predetermined width between itself and gate electrode 161; the silicon layer 120 patterned to be orthogonal to gate electrode 161; contact holes 220 formed through to gate electrode 161 and a gate line electrodes 160[D₁ ] and 160[D₂ ] and wherein metal lines 200 and source electrode 200-1 and drain electrode 200-2 are formed to have a width narrower than the gate line electrodes 160[D₁ ] and 160[D₂ ] so as to connect the gate electrode to the gate line electrodes 160[D₁ ] and 160[D₂ ].

As taken along line B-B' in FIG. 6, the silicon layer 120 formed over transparent substrate 100, as shown in FIG. 7. First insulating layer 140 is formed over silicon layer 120. The gate electrode 161 is formed on first insulating layer 140 and second insulating layers 240 is formed over first insulating layer 140. When formed the upper surface of and the side surfaces of gate electrode 161 are exposed.

Metal line 200 is formed on second insulating layers 240 to be narrower than the gate electrode 161. The source 200-1 and drain 200-2 metal electrode is formed through the source-drain contact hole.

As taken along lines A-A' of FIG. 6, silicon layer 120 is formed over transparent substrate 100 and patterned into islands separated from each other by a predetermined space, as shown in FIG. 8. First insulating layer 140 is formed over silicon layer 120 and patterned with silicon layer 120. Gate electrode 161 and gate line electrodes 160[D₁ ] and 160[D₂ ] are formed on first insulating layer 140. Second insulating layer 240 is formed over the resulting structure overlaying transparent substrate 100. Metal line 200 is formed to have a predetermined thickness on second insulating layers 240.

When it comes to comparing the sectional view of FIG. 8 and the conventional one of FIG. 1C, a point "I" indicated in FIG. 8 is wider than the point "I" indicated in FIG. 1C. That is, the thickness "I" of the insulating layer of FIG. 1C is about 1000 angstroms, and the thickness "I" of the insulating layer of FIG. 8, may be formed to be about 6000 angstroms. This may solve the problem of deterioration in the performance characteristics of the associated transistors, and increase production yield, since failures due to excessive leakage current may be avoided.

According to the present invention, the oxide layer is formed right after the deposition of the silicon to reduce impurities and is formed in such a manner as it alleviates problems caused by leakage current in the conventional transistor structure. Moreover, simplification of the manufacturing process, and improved reliability of the resulting LCD may be obtained by use of the present invention. 

What is claimed is:
 1. A method of manufacturing thin film transistor for use in a liquid crystal display, comprising the steps of:providing a transparent substrate; forming a silicon layer over the substrate; forming a first insulating layer over the silicon layer, said first insulating layer having a first predetermined thickness; forming a gate electrode layer over the first insulating layer; patterning the gate electrode layer to form a plurality of substantially parallel, linear, and non-contiguous gate lines having a first width, each said gate line comprising an associated pair of gate line electrodes each separated by a predetermined distance from a gate electrode positioned therebetween; patterning the silicon layer into a plurality of thin-film transistor regions; forming a source region and a drain region in each said thin-film transistor region; forming a second insulating layer over the plurality of gate electrodes and the associated pairs of gate line electrodes, said second insulating layer having a second predetermined thickness greater than said first predetermined thickness; selectively forming a plurality of first contact holes through the second insulating layer to expose a portion of the plurality of gate electrodes and the associated pairs of gate line electrodes; and, forming a plurality of metal lines having a second width, each said metal line ohmically connected through the first contact holes to the gate electrode and the associated pair of gate line electrodes within a gate line.
 2. The method of claim 1, in which the second width is less than the first width.
 3. The method of claim 2, further comprising the steps of:selectively forming a plurality of second contact holes in the second insulating layer to expose portions of the source and drain regions; and, forming source and drain electrodes through the plurality of second contact holes, whereby said source and drain electrodes are ohmically connected to said source and drain regions, respectively.
 4. The method of claim 3, wherein said source and drain electrodes are formed substantially orthogonal to said gate lines.
 5. The method of claim 4, wherein said gate lines are polysilicon.
 6. The method of claim 5, wherein said first insulating layer is formed by a thermal oxide process.
 7. The method of claim 5, wherein said first insulating layer is formed by a chemical vapor deposition process.
 8. A method of manufacturing a thin film transistor for use in a liquid crystal display, comprising the steps of:providing a transparent substrate; forming a silicon layer over the substrate; forming a first insulating layer over the silicon layer, said first insulating layer having a first predetermined thickness; forming a gate electrode layer over the first insulating layer; patterning the gate electrode layer to form a plurality of substantially parallel, linear, and non-contiguous gate lines having a first width, each said gate line comprising an associated pair of gate line electrodes each separated by a predetermined distance from a gate electrode positioned therebetween; patterning the silicon layer into a plurality of thin-film transistor regions; forming a source region and a drain region in each said thin-film transistor region; forming a second insulating layer over the substrate, the plurality of thin-film transistor regions, and the plurality of gate lines, said second insulating layer having a second predetermined thickness greater than said first predetermined thickness; selectively forming a plurality of first and second contact holes through the second insulating layer, whereby said plurality of first contact holes expose portions of the plurality of gate electrodes and the associated pairs of gate line electrodes, and said plurality of second contact holes expose portions of said source and drain regions; and, forming a plurality of first and second metal lines over said second insulating layer and in said plurality of first and second contact holes, respectively, whereby each said first metal line has a second width and is ohmically connected through the first contact holes to the gate electrode and the associated pair of gate line electrodes within a gate line, and said plurality of second metal lines comprise source and drain electrodes which are ohmically connected through said second contact holes to said source and drain regions, respectively.
 9. The method of claim 8, wherein said source and drain electrodes are formed substantially orthogonal to said gate lines.
 10. The method of claim 9, wherein said gate lines are polysilicon.
 11. The method of claim 10, wherein said first insulating layer is formed by a thermal oxide process.
 12. The method of claim 10, wherein said first insulating layer is formed by a chemical vapor deposition process.
 13. A method of manufacturing a thin film transistor for use in a liquid crystal display, comprising the steps of:providing a transparent substrate; forming a silicon layer over the substrate; forming a first insulating layer over the silicon layer, said first insulating layer having a first predetermined thickness; forming a gate electrode layer of polysilicon over the first insulating layer; patterning the gate electrode layer to form a plurality of substantially parallel, linear, and non-contiguous gate lines having a first width, each said gate line comprising an associated pair of gate line electrodes each separated by a predetermined distance from a gate electrode positioned therebetween; patterning the silicon layer into a plurality of thin-film transistor regions; forming a source region and a drain region in each said thin-film transistor region; forming a second insulating layer over the substrate, the plurality of thin-film transistor regions, and the plurality of gate lines, said second insulating layer having a second predetermined thickness of approximately 4000 to 6000 angstroms; selectively forming a plurality of first and second contact holes through the second insulating layer, whereby said plurality of first contact holes expose portions of the plurality of gate electrodes and the associated pairs of gate line electrodes, and said plurality of second contact holes expose portions of said source and drain regions; and, forming, substantially orthogonal to said gate lines, a plurality of first and second metal lines over said second insulating layer and in said plurality of first and second contact holes, respectively, whereby each said first metal line has a second width less than said first width and is ohmically connected through the first contact holes to the gate electrode/and the associated pair of gate line electrodes within a gate line, and said plurality of second metal lines comprise source and drain electrodes which are ohmically connected through said second contact holes to said source and drain regions, respectively. 